Bump future_rel and changes_future to 8.0 and changes_cur to 7.0.
The user-visible change is that instead of having `NetBSD/<port> 6.0 changes'
and `NetBSD/<port> 7.0 changes' there will be respectively
`NetBSD/<port> 7.0 changes' and `NetBSD/<port> 8.0 changes'.